In recent years, reduction of the weight of automobiles has been strongly demanded from the viewpoint of the global environment. In automobile bodies, for example, pillars, door impact beams, bumper beams, and other structural parts for automobiles, high strength steel sheet is being used to reduce the thickness of steel sheet to try to lighten the weight. For this reason, the strength of steel sheet is being raised. In particular, high strength steel sheet with a tensile strength (TS) over 1000 MPa is being developed, but higher strength of steel sheet leads to a drop in the workability and press formability at the time of production of a part. In particular, it becomes more difficult to ensure product precision due to springback etc.
To solve these problems, in recent years, as a technique for simultaneously satisfying higher strength and workability of the steel sheet and product precision, the hot stamping method (press quenching method) has come to be used as a practical method. For example, this is disclosed in PLT 1. This heats steel sheet to an approximately 900° C. or so austenite region, then press forms it hot and, at the time of press forming, brings it into contact with an ordinary temperature die set to quench it and thereby obtain a high strength material. Due to this hot stamping method, the residual stress which is introduced at the time of press forming is also reduced, so the inconveniences of fracture, poor shape freezing, etc. which become problems in high strength steel sheet with a TS of over 1180 MPa are suppressed and production of parts with relative good product precision becomes possible.
In the high strength steel sheet which is used for automobiles etc., the above-mentioned problems become more serious the higher the strength. Further, in particular, in high strength materials of over 1000 MPa, as has been known in the past, there is the inherent problem of hydrogen embrittlement (also called “season cracking” or “delayed fracture”). In the case of steel sheet for hot pressing use, while the residual stress due to pressing at a high temperature is small, hydrogen penetrates the steel at the time of heating before pressing and the susceptibility to hydrogen embrittlement becomes higher due to the residual stress after pressing.
As the method of preventing cracking due to delayed fracture, there is the method of controlling the heating atmosphere at the time of hot stamping. For example, PLT 2 proposes the method of making the hydrogen concentration in the heating atmosphere of the hot stamping 6 vol % or less and making the dew-point 10° C. This relates to a method of control of the heating atmosphere of hot stamping. That is, by controlling the hydrogen concentration and the condensation point, the penetration of external hydrogen into the steel sheet during heating is suppressed. Therefore, this does not improve the steel sheet itself. It can only be applied in hot stamping which has a system for controlling the atmosphere.
In addition, as the steel sheet for hot stamping use, there is known steel sheet which traps the hydrogen which penetrates the steel sheet and thereby prevents delayed fracture. For example, PLT 3 proposes steel sheet for hot stamping use which improves the delayed fracture resistance. This art incorporates average particle size 0.01 to 5.0 μm range Mg oxides, sulfides, composite crystals, and composite precipitates, e.g. one or more composite oxides among them, into the steel in an amount of 1×102 to 1×107 per square mm. These oxides and composite crystals and composite precipitates having these as nuclei act as hydrogen trap sites to thereby improve the delayed fracture resistance.
Further, as similar art, PLT 4 discloses the art of producing high strength thin-gauge steel sheet which is excellent in hydrogen embrittlement resistance characterized by making bainite or martensite the biggest phases in terms of area rate, making one or more of Nb, V, Cr, Ti, and Mo oxides, sulfides, nitrides, composite crystals, and composite precipitates in the particles satisfy an average particle size “d”: 0.001 to 5.0 μm, a density ρ: 100 to 1×1013/mm2, and a ratio of standard deviation σ of average particle size and average particle size “d”: σ/d≤1.0, and by having a tensile strength of 980 MPa or more.
Furthermore, in steel sheet for enameling use, to improve the fishscale susceptibility, it is known that it is effective to form voids in the steel sheet to trap the hydrogen. PLT 5 proposes to form Fe—Nb—Mn-based composite oxides in steel sheet and increase the segregation of Nb and Mn in the oxides so as to raise the hydrogen trapping ability. However, the art which is described in PLT 5 is art which assumes steel sheet for enameling use which has a small C (carbon) content (usually 0.01 mass % or less). In large C content high strength steel sheet (C of 0.05 mass % or more) such as steel sheet for automobile use, the oxidizing action of C cannot be ignored. Therefore, this cannot be simply applied.
Further, the amount of hydrogen problematic in steel sheet for enameling use is a high concentration of 10 to 100 ppm, while with high strength steel sheet, an amount of hydrogen of a very low concentration of 1 to 3 ppm is considered a problem.
Therefore, the art which is described in PLT 5 cannot be applied as is to large C content high strength steel sheet.
To apply these arts to large C (carbon) content high strength steel materials, suitable control of the size (average particle size) and presence (density) of the oxides etc. present in the steel sheet is an important requirement. However, strict control to give a particle size and density which are effective as hydrogen trap sites and which do not form starting points of coarse cracks is not technically easy.